Unfortunaly there is no MWE in the question. So I do not know which KOMA-Script class is used and if the document is onesided or twosided. So here is an example for twosided scrbook using geometry and scrlayer-scrpage. If you use the outdated package scrpage2 or fancyhdr (not recommended for use with KOMA-Script) see here.
\documentclass{scrbook}[2015/10/03]
\usepackage{blindtext}% dummy text
\usepackage[T1]{fontenc}
\usepackage{geometry}
\usepackage{calc}
\newcommand\LeftMargin{3cm}
\geometry{
layoutsize={21cm-\LeftMargin,27.9cm},
layoutoffset={\LeftMargin,0cm},
reversemarginpar,
inner=2cm,
outer=4cm,
top=3cm,
bottom=4cm,
heightrounded,
%showframe% to show the pagelayout
}
\makeatletter
\renewcommand{\chapterlinesformat}[3]{%
\hspace*{-\LeftMargin}%
\parbox[t]{\textwidth+\LeftMargin}{\raggedchapter\@hangfrom{#2}{#3}}%
}
\renewcommand{\sectionlinesformat}[4]{%
\hspace*{-\LeftMargin}%
\parbox[t]{\textwidth+\LeftMargin}{\raggedsection\@hangfrom{#3}{#4}}%
}
\makeatother
\usepackage[
headwidth=\the\textwidth+\LeftMargin:-\LeftMargin:\LeftMargin% enlarge the headwidth
,headsepline,plainheadsepline% to make the header visible
,footwidth=head:-\LeftMargin:\LeftMargin% enlarge the footwidth
,footsepline,plainfootsepline% to make the footer visible
]{scrlayer-scrpage}
\begin{document}
\tableofcontents
\blinddocument
\end{document}

If you are using scrartcl remove the lines which redefines \chapterlinesformat and change the geometry settings because scrartcl is onesided.

Code:
\documentclass{scrartcl}[2015/10/03]
\usepackage{blindtext}% dummy text
\usepackage[T1]{fontenc}
\usepackage{geometry}
\usepackage{calc}
\newcommand\LeftMargin{3cm}
\geometry{
layoutsize={21cm-\LeftMargin,27.9cm},
layoutoffset={\LeftMargin,0cm},
reversemarginpar,
left=3cm,
right=3cm,
top=3cm,
bottom=4cm,
heightrounded,
%showframe% to show the pagelayout
}
\makeatletter
\renewcommand{\sectionlinesformat}[4]{%
\hspace*{-\LeftMargin}%
\parbox[t]{\textwidth+\LeftMargin}{\raggedsection\@hangfrom{#3}{#4}}%
}
\makeatother
\usepackage[
headwidth=\the\textwidth+\LeftMargin:-\LeftMargin:\LeftMargin% enlarge the headwidth
,headsepline,plainheadsepline% to make the header visible
,footwidth=head:-\LeftMargin:\LeftMargin% enlarge the footwidth
,footsepline,plainfootsepline% to make the footer visible
]{scrlayer-scrpage}
\begin{document}
\tableofcontents
\blinddocument
\end{document}